This invention relates to the determination of the quantity of a first component in a solution that contains a second component of appreciably higher electrical conductivity, and is particularly, though not exclusively, relevant to the determination of the quantity of silver in a photographic fixing solution.
Although the invention is more generally applicable, for convenience of explanation only, it will be described with reference to photographic processing.
The processing of photosensitive material, film or paper, entails the material being passed sequentially through baths containing developing, bleach and fixing solutions. Silver halides are produced in the developing step, and the fixing step converts these into silver ion complexes that are soluble in water, and which may be removed in a subsequent washing step. Thiosulphates, of sodium or ammonium, are the commonly-used fixing agents. As the concentration of the dissolved halides increases, the time needed to fix the image in the material increases. In order to obtain uniform processing conditions, in a mini-lab for example, it is desirable to maintain the fixing time substantially constant. To this end, the fixer is replenished with fresh solution from time-to-time, usually in dependence on the area of material that has been processed. The effluent from the processing thus contains a quantity of silver ions. Environmental and cost considerations require that these components are not simply fed as waste to a drain. It is known to supply such effluent to apparatus for removing the silver from the solution, which is then recovered for further use.
Problem to be Solved by the Invention
A silver recovery unit operates efficiently only when the concentration of silver in the solution is above a defined minimum value. Thus, it is known to measure the quantity of silver in solution, for example in the fixer bath, and to operate the unit only under efficient conditions. Accurate determination of the quantity of silver ions in a fixer solution has hitherto been difficult precisely due to the presence of the thiosulphate, which is itself electrically conductive.
It will be appreciated that a similar problem can arise in other applications.
In accordance with one aspect of the present invention, there is provided a method of determining the quantity of a first component in a solution that contains a second component of appreciably higher electrical conductivity, the method comprising the steps of:
(a) measuring the electrical conductivity of the solution;
(b) measuring the electrical potential between two electrodes immersed in the solution; and
(c) applying a predetermined algorithm to the measured values of conductivity and potential, thereby to determine the said quantity.
Electrical conductivity is considered to be xe2x80x9cappreciably higherxe2x80x9d if measurement of the conductivity of the solution as a whole can be taken as being substantially the conductivity of the second component alone.
Preferably, the measurements of steps (a) and (b) are made sequentially, thereby eliminating any interference between the two measurements.
The said quantity of the first component may comprise the concentration of silver ions, and the solution may comprise a photographic fixing agent, preferably a thiosulphate, of sodium or ammonium for example. Particularly, though not exclusively, in this case, one of the electrodes may be a silver electrode and the other electrode may be a reference electrode. A suitable silver electrode, as commonly used in pH measurements, is disclosed, for example, in the book xe2x80x9cpH Measurementxe2x80x94Fundamentals, Methods, Applications, Instrumentationxe2x80x9d by Helmuth Galster (VCH 1991).
The method of the invention may comprise the steps of supplying a signal in accordance with the said quantity to apparatus for recovering said first component from the solution, and operating the recovery apparatus only when that quantity has a predetermined value, preferably when said quantity is above a predetermined minimum value.
The particular algorithm to be applied to the measured values is to be determined empirically, in dependence on the concentration of the thiosulphate and of the silver ions. An example for a typical arrangement for measuring the quantity of silver is:       V    Ag    =            a      ⁢              xe2x80x83            ⁢              ln        ⁢                  (                      Ag            +                    )                      +          (                                    C            d                    b                -        c            )      
where,
VAg is the electrical potential measured between the two electrodes in the solution,
Ag+ is the concentration of silver ions,
Cd is the conductivity of the solution, and
a, b and c are known constants.
The steps of the method may be carried out continuously, or alternatively only from time-to-time as required.
In accordance with a further aspect of the present invention, there is provided apparatus for determining the quantity of a first component in a solution that contains a second component of appreciably higher electrical conductivity, comprising:
(a) a vessel for containing the solution;
(b) means for measuring the electrical conductivity of the solution;
(c) two electrodes arranged to be immersed in spaced-apart relationship in the solution in the vessel;
(d) means for measuring the electrical potential between the two electrodes; and
(e) means for receiving signals in accordance with the measured values of conductivity and potential, said receiving means being arranged to determine the said quantity of the first component in the solution.
Preferably, and particularly for the recovery of silver from a solution, one of said electrodes is a silver electrode and the other electrode is a reference electrode.
Advantageously, the apparatus comprises apparatus for recovering the first component from the solution, said recovery apparatus being arranged to receive the solution from the vessel, the recovery apparatus being arranged to be operated only on receipt of a signal from the receiving means indicating that the quantity of said first component has a predetermined value, preferably when said quantity is above a predetermined minimum value.
Advantageous Effect of the Invention
Hitherto, the quantity of silver in the fixer solution has been determined from the potential measurement alone. In accordance with the present invention, a correction is made to this measurement to take into account the conductivity of the thiosulphate. The resulting more accurate measurement of the amount of silver in the solution allows a finer control of the subsequent silver recovery unit, that is to say, allows the unit to be operated for a longer time on a given solution. This improved efficiency results not only in the cost saving of recovering more silver, but also in a correspondingly cleaner effluent from the unit, which may be disposed of more conveniently. It has been found that using the present invention, the concentration of silver ions in a fixer solution can be determined to within xc2x110%.